1. Technical Field
The present invention relates to an antenna apparatus applied to a radar apparatus which detects an azimuth of a target.
2. Related Art
Conventionally, a so-called phase mono-pulse method radar apparatus is known in which a plurality of (multiple-channel) receiving antennas receive waves which are radiated from a transmitting antenna and reflected from a target. The radar apparatus detects an azimuth of the target based on phase differences between signals of respective channels received by the receiving antennas.
However, when detecting the azimuth by using the phase difference, so-called phase wrap around (grating) is generated based on the fact that a phase difference Δφ and Δφ±360n° (n is a natural number) cannot be distinguished from each other. Hence, due to the false image generated by the phase wrap around, a target located outside a range, in which the phase difference changes by 360° (e.g. −180°<Δφ≦180° of the range of the azimuth), is erroneously detected as a target which is located within the range.
To solve the above problem, various methods are proposed for preventing erroneous detection due to the phase wrap around. For example, in JP-A-2010-32314, a technique is disclosed which suppresses the grating by unequalizing intervals between the plurality of receiving antennas. In addition, in JP-A-2009-76986, a technique is disclosed in which adjacent receiving antennas of a plurality of receiving antennas share an array which forms each of the receiving antennas.
However, according to the technique disclosed in JP-A-2010-32314, since the receiving antennas are arranged at unequal intervals, a unintended and undesired wave is generated under the specific condition. According to the technique disclosed in JP-A-2009-76986, part of a plurality of arrays forming a receiving antenna are shared by another adjacent receiving antenna, isolation between the adjacent receiving antennas deteriorates. In addition, since the part of the arrays is shared, designing and manufacturing the antenna becomes complicated.
Note that, to prevent the erroneous detection due to the phase wrap around, the interval between receiving antennas of respective channels is shortened in general. From a viewpoint of the resolution for detecting an azimuth angle, the width of the whole receiving antenna is preferred to be larger. However, to prevent the erroneous detection due to the phase wrap around while realizing high resolution, the number of the receiving antennas (the number of channels) is required to be larger, which increases the manufacturing cost.